This invention relates to a broad-band radio wave absorber useful for constructing anechoic chambers.
An anechoic chamber is now widely used for performing a variety of tests such as for undesirable radiation (noise) from electronics apparatuses, for electromagnetic obstruction, for electromagnetic compatibility and for antenna characteristics. Such an anenchoic chamber is provided with wave absorbers on the inside walls and ceilings thereof.
One known radio wave absorber is shown in FIG. 23 in which designated as M is a conductive metal plate for reflecting a radio wave and as F a sintered ferrite plate in the form of a tile mounted on the metal plate M. In the meantime, when the reflection coefficient at a surface of the wave absorber is represented by "s", the power absorption coefficient thereof is given by 1-.vertline.s.vertline..sup.2. Thus, the smaller the reflection coefficient .vertline.s.vertline., the better becomes the absorber performance. Generally, an absorber having a reflection coefficient .vertline.s.vertline. of 0.1 or less is regarded as meeting with the standard. In other words, the standard requires that the return loss (-20 log s) should be 20 dB or more and the power absorption coefficient should be 0.99 or more.
FIG. 24 shows the characteristics of the wave absorber of FIG. 23. In FIG. 24, the abscissa represents frequency f while the ordinate represents reflection coefficient .vertline.s.vertline.. As seen from FIG. 24, the band width B which satisfies the condition .vertline.s.vertline..ltoreq.0.1 may be given as follows: EQU B=f.sub.H -f.sub.L ( 1)
wherein f.sub.L and f.sub.H represent the lowest and highest frequencies at which .vertline.s.vertline. is 0.1, respectively. In the wave absorber shown in FIG. 23, the frequencies f.sub.L and f.sub.H depend upon the ferrite material used. For example, when desired f.sub.L is 30 MHz, sintered ferrite of a NiZn-series or MnZn-series must be used. In this case, f.sub.H is 300-400 MHz. When f.sub.L of 90 MHz is desired, then the ferrite to be used is of a NiZn-series or MnZn-series. In this case, f.sub.H is 350-520 MHz. Since an anechoic chamber requires a wave absorber having f.sub.L of 30 MHz and f.sub.H of 1,000 MHz, the wave absorber of FIG. 23 is not suited therefor. Further, the wave absorber of FIG. 3 is ill-suited for use as an exterior wall material of buildings for the prevention of reflection of TV radio waves, when the required f.sub.L and f.sub.H are 90 MHz and 800 MHz, respectively, like in Japan.
To cope with this problem, there is a proposal in which an air layer (e.g. polyurethane foam layer) is interposed between the ferrite tiles F and the metal plate M in FIG. 23. A wave absorber composed of 7 mm thick NiZn ferrite tiles mounted on the metal plate through an 10 mm thick air layer, for example, shows a return loss of 20 dB or more for a radio wave having a frequency range of 30-800 MHz.
U.S. Pat. No. 5,276,448 discloses a wave absorber of a lattice structure as shown in FIGS. 25(a) and 25(b). This wave absorber shows a return loss of 20 dB or more for a radio wave of 30-1,000 MHz when a lattice-type ferrite plate F mounted on a metal plate M has a thickness t.sub.m of 7 mm and a height h of 18 mm and, thus, exhibits satisfactory wave absorbing performance. In recent years, an increasing attention has been paid to an importance of electromagnetic immunity of electronic instruments. Because the frequency of radio waves generated from recent electronic instruments widely ranges, there is an increasing demand for wave absorbers having a high f.sub.H. In this respect, the above lattice structure-type wave absorber is not satisfactory.
Japanese Unexamined Patent Publication 5-82995 discloses a wave absorber of a superimposed lattice structure as shown in FIGS. 26(a) and 26(b). This absorber has f.sub.L of 30 MHz and f.sub.H of 3,000 MHz and is effective for a broad band of frequencies. The superimposed lattice structure-type wave absorber, however, has a problem because of difficulty in manufacture. In particular, it is very difficult to prepare the structure, in which the top ferrite has a thickness t.sub.m3 of less than 1 mm, by molding, due to poor flowability of the powder mass, non-uniformity in molding pressure and poor mold-releasability.